The corrosion protection of steel substrates is important for many industries, including the automotive and steel industries. Currently the most common methods of corrosion protection of steel substrates are galvanizing, application of zinc phosphate, application of primer materials by electrodeposition, conventional spray or dip priming, oil coating and combinations thereof. However, especially in the automotive industry, these methods are associated with pollution in the form of volatile organic compounds (VOC), (2) excessive waste disposal, (3) inadequate coverage of recessed areas, and (4) inadequate retention or performance of corrosion protection.
It is generally known that plasma deposition of thin films gives a very dense layer of film, with uniform deposition, no "pin holes", and good edge coverage. Furthermore, such a process does not require solvents, so there is no VOC problem. However, most of the work in the plasma deposition area has been restricted to small objects (e.g. microelectronic components). Plasma processing for larger objects has been used primarily for plastic substrates.
The plasma deposition of organic films on metal is generally described in an article entitled "Surface Coating of Metals in a Glow Discharge" in the Journal of the Oil and Colour Chemists Association, Vol. 48, 1965 (hereinafter, the "Glow Discharge" article). This article describes, in general terms, a method of coating a steel substrate with thin polymer films derived from organic vapors (styrene, acrylates, butadiene, diethyl silicate, and tetraethyl orthosilicate) using glow discharge (i.e., plasma deposition) for short term protection of the steel substrate.
The plasma deposition of organosilanes and other thin films for corrosion protection of steel in the automotive industry is disclosed in U.S. Pat. No. 4,980,196. In the processes disclosed therein, multi-layered coatings are formed, including the combination of a thin film, by means of plasma deposition, with a primer coating. In Example 5 of the patent, an electrocoat primer was employed. However, in order to be competitive with present processes, there is a need for even better adhesion and/or corrosion protection.
What is needed is an improved method of providing corrosion resistance, particularly of metal substrates involved in automobile production. Such an improved method must result in a coating having good adhesion, good edge coverage, and good barrier properties. It would be especially desirable to be able to obtain improved corrosion protection of metal that has not been galvanized, which corrosion protection is comparative or better than existing processes involving galvanized metals. For example, a method which uses bare or cold rolled steel instead of galvanized steel would be advantageous, not only because the metal substrate is less expensive and easier to manufacture, to begin with, but because material recycling of parts or, eventually, a used automobile is significantly more expensive if the metal has been galvanized.